As a semiconductor material where lowering of electron mobility in a high electric field may be suppressed, a wide band gap semiconductor has been used. For example, in a semiconductor device which uses a nitride semiconductor, an electron current generated by a two-dimensional electron gas may be controlled based on a potential of a gate electrode. Agate insulation film is interposed between the gate electrode and the nitride semiconductor. In such a semiconductor device, a highly reliable gate insulation film is required for achieving operation of the semiconductor device at a high voltage.
In manufacturing, the material from which the gate electrode is formed is disposed on the gate insulation film, and the gate electrode material is pattern etched using reactive ion etching to define the gate electrode out of the gate electrode material layer. However, when forming the gate electrode by plasma etching, the gate insulation film is exposed to ion bombardment from the plasma thus giving rise to a possibility that a defect occurs in the gate insulation film. In this case, the insulation property of the gate insulation film is lowered.